1. Field of the Invention
This invention relates to synthetic resins, and more particularly to processes for casting polyester resin.
2. Description of the Prior Art
Cast unsaturated polyesters using inert fillers is an art which has been practiced commercially since the early 1960's. Products are known variously as cultured marble, cast marble, precast marble and molded marble. In the 1970's, a synthetic onyx was developed using different fillers. Products in both marble and onyx include table tops, lamp bases, window sills, countertops, wall panels and various items of sanitary-ware such as vanity tops, bathtubs and showers.
Typically, cast marble consists of 20-35% unsaturated polyester resin such as, for example, propylene glycol esterified with adipic and maleic anhydride and filled with an inert material such as calcium carbonate. Cast onyx consists of a specially clarified polyester resin with glass frit and alumina trihydrate used either separately or in combination as the filler. The conventional manufacturing process consists of the following steps:
(1) preparing the molds by covering the surface with a suitable mold release agent, typically a wax or organic alcohol.
(2) spraying the mold surface with a clear, unsaturated polyester resin referred to as a gel coat into which has been incorporated an ultraviolet stabilizer and a thixotrope. An organic peroxide is used as catalyst.
(3) after the gel coat has partially cured, a matrix is troweled or poured into the mold. The matrix is prepared by mixing polyester resin, organic peroxide catalyst, filler and colorants in the desired proportions.
(4) after the molds are filled, they are vibrated for several minutes to try to effect air release from the matrix, but especially from the interface with the gel coat.
(5) the material is then allowed to cure at ambient temperature to a hard and "rigid" state before removal of the formed part from the mold.
A typical example--U.S. Pat. No. 3,562,379 to Duggins--discloses an improved cast marble polyester resin process. U.S. Pat. No. 3,562,379 and U.S. Pat. No. 3,488,346 are also examples of the advanced state of the art.
Cast marble has only one real competitor. In the 1960's, du Pont developed an alumina trihydrate filled methylmethacrylate polymer which it has marketed under the mark "Corian". In competing with methylmethacrylate polymer, cast marble has had several significant advantages. It can be manufactured at a lower cost, it has considerably greater versatility as to colors, shapes and sizes, and its physical properties are better. In addition, it is more satisfactorily repairable.
The major disadvantage of cast marble has been its lack of machineability because of the porosity of the matrix resulting from incomplete removal of entrapped air during the casting operation. Vibrating, as practiced conventionally, is inadequate to remove the air. This has effectively ruled out cast marble in the kitchen countertop market where traditionally the countertops are field cut to specific requirements.
De-gassing agents such as disclosed in U.S. Pat. No. 4,129,543 to Kaplan have been used, but with limited success due to the high viscosity of the resin which inhibits air release. Further, they are expensive in the large quantities called for in, for example, the Kaplan patent and which quantities have heretofore been necessary to achieve even limited success and which, according to the Kaplan patent, may result in surface charring.
Evacuating the blend in the mold tends to devolve styrene monomer preferential to degassing the air and, further, it is not economically feasible to put the large molds common to the industry in vacuum chambers.
There is, therefore, a need for a simple, economical cast marble process which will remove entrapped air from a polyester resin to leave a uniformly dense matrix and to accomplish this end result without deliterious effects on the polymerization process itself.